The invention relates to an optically variable see-through security element for securing value objects, with a flat, optically variable area pattern which in transmission shows a colored appearance with a viewing-angle-dependent, polychrome color change.
Data carriers, such as value documents or identification documents, but also other value objects, such as branded articles, are frequently provided for securing purposes with security elements which permit a check of the authenticity of the data carrier and which at the same time serve as protection against unauthorized reproduction. Here, see-through security features, such as see-through windows in banknotes, are becoming increasingly attractive.
Conventional transparent or semitransparent security elements with a viewing-angle-dependent, polychrome color change in transmitted light have various disadvantages, however. Thus, it is known for example to produce diffraction colors in transmitted light with transparently or semitransparently coated hologram gratings or transmission gratings, wherein it can be achieved by suitable choice of the grating periods and the azimuth angles of the gratings that different representations with changing colors emerge at different viewing angles. The appearance of such grating images, however, strongly depends on the lighting conditions. When illuminated with a point light source, individual subregions can flash very brightly and disappear quickly again at certain angles, while in diffuse ambient light only a very weak or possibly even no diffraction effect may be visible. Also, the perceived color does not only depend on the viewing angle to the security element, but also on the direction to the light source, wherein in addition a corresponding security element must not be held directly in front of a light source for viewing the diffraction colors of the first order, but the security element must be held somewhat out of the direct connecting line. Further, upon tilting the security element all rainbow colors are run through, so that the color changes occurring are largely undefined and the observed color effects are frequently perceived as simply colorful by the untrained viewer. Finally, holographic techniques have become common also outside the security sector and therefore now offer only a limited protection against imitation.
In a different solution, colors are produced with thin film systems through interference in incident light and in transmitted light, which colors change in dependence on the viewing angle. Different colors are therein usually realized by a variation of the layer thicknesses, for example the thickness of a dielectric spacer layer in a three-layer structure of absorber/dielectric/absorber. The adjustment of a desired color by adjusting the layer thicknesses is technologically very elaborate, however. One possibility is the regional printing of one or a plurality of dielectric layers, however very high demands are placed on the uniformity of the printed layers and the lateral resolution is limited to the resolution attainable by the corresponding printing methods. Moreover, motif changes upon tilting can practically not be implemented with such thin film systems.
A further solution is to produce colors in incident light and in transmitted light with transparently or semitransparently coated subwavelength structures, which colors change upon tilting the structures. Such subwavelength structures are very challenging to produce and difficult to manufacture on the required industrial scale, however.
Proceeding therefrom, it is the object of the present invention to specify a see-through security element of the type mentioned at the outset that avoids the disadvantages of the state of the art. In particular, the see-through security element is to combine an appealing visual appearance with high falsification security, and ideally be manufacturable on the industrial scale required in the security sector.